Role of prior austenite grain structure in hydrogen diffusion, trapping, and embrittlement mechanisms in as-quenched martensitic steels

Prior austenite grain structure (PAG) is an essential factor in martensitic steels that affects hydrogen (H) diffusion, trapping, and susceptibility to hydrogen embrittlement (HE). The influence of PAG morphology on HE susceptibility of ultrahigh-strength steels has been previously studied with a novel tuning-fork test (TFT). To achieve different PAG morphologies with the same alloying composition, a direct-quenched steel (DQ) was reaustenitized at 860°C (A860) and 960°C (A960) for 25 min, followed by quenching. DQ and A860 have different PAG morphologies, elongated vs. equiaxed, but similar ∼10 µm average PAG size. A860 and A960 have the same equiaxed morphology but a fourfold difference in PAG size. To evaluate the TFT method, and to provide a further understanding of the effect of PAG structure on H diffusion and trapping, in-situ constant load tensile tests (CLT), electrochemical hydrogen permeation (EP), and thermal desorption spectroscopy (TDS) measurements were conducted with the same materials. CLT produced the same results as TFT, where the original DQ material with elongated PAG structure has the best resistance against HE with the longest time-to-fracture and a quasi-cleavage crack propagation mechanism. A860 and A960 with equiaxed PAG structures are more susceptible to HE, showing partly intergranular crack propagation, linking to the geometrical shape of the PAG structure. The diffusion of H is here dominated by the simultaneous effects of the PAG surface area and dislocation density. Therefore, H diffusion is the slowest for DQ with a slight increase for A860 and A960.